Low birth weight is associated with an increased risk for cardiovascular disease. Our laboratory has developed a unique model of placental insufficiency in the rat that results in intrauterine growth restriction (IUGR) associated with hypertension in male IUGR, but not in female IUGR. Importantly, castration abolishes hypertension in male IUGR; ovariectomy (OVX) induces a marked increase in blood pressure (BP) in female IUGR. Thus, sex hormones may mediate differences in adult IUGR BP; yet, the exact mechanism(s) remains unknown. Preliminary data suggest that renal endothelin (ET) is increased in male IUGR and in response to OVX in female IUGR; hypertension in male IUGR and OVX female IUGR is abolished by selective ETA receptor blockade. Therefore, modulation of ET by sex hormones may be critical to sex differences in IUGR BP. Preliminary data suggest oxidative stress is increased in male IUGR and OVX female IUGR. Chronic treatment with tempol, a SOD mimetic, abolishes increased oxidative stress and hypertension in male IUGR and female OVX IUGR. We report that the renin angiotensin system (RAS) plays a critical role in hypertensive IUGR; however, peripheral and intrarenal levels of the RAS are not elevated. ET can increase responsiveness to angiotensin II (ANG II); preliminary data suggest responsiveness to ANG II is increased in male IUGR and OVX female IUGR. Thus, ET induced oxidative stress and vascular responsiveness to ANG II may serve as possible mechanisms by which ET contributes to sex differences in IUGR BP. Thus, we hypothesize that testosterone increases renal vascular resistance and exacerbates hypertension in male IUGR by enhancing the endothelin system, thus leading to increased oxidative stress and vascular responsiveness to angiotensin II; and that estradiol decreases renal vascular resistance and blood pressure in female IUGR by attenuating the endothelin system, thus preventing an increase in oxidative stress and vascular responsiveness to angiotensin II. We will test the following specific aims: Specific aim 1: That testosterone further increases renal vascular resistance and exacerbates hypertension in male IUGR by enhancing the endothelin system; and that estradiol reduces renal vascular resistance and blood pressure in female IUGR by attenuating the endothelin system. Specific aim 2: That testosterone-mediated enhanced endothelin production increases oxidative stress leading to a further increase in renal vascular resistance and exacerbating hypertension in male IUGR; and that estradiol-mediated attenuation of endothelin production decreases oxidative stress leading to a reduction in renal vascular resistance and blood pressure in female IUGR. Specific aim 3: That testosterone-mediated enhanced endothelin production increases vascular responsiveness to angiotensin II further increasing renal vascular resistance and exacerbating hypertension in male IUGR; and that estradiol- mediated attenuation of endothelin production decreases the vascular responsiveness to angiotensin II leading to a reduction in renal vascular resistance and blood pressure in female IUGR.